There is a need for devices or apparatus, which make it possible to identify or detect items or objects in a contactless manner. Such devices or apparatus may be manufactured integral with or attached to the object being identified. It is also important that such devices operate effectively and accurately within a selected distance from an interrogation unit and that the device be inexpensive.
More specifically, it is, for example, desirable to receive the unique identification, which is assigned to an object and which is stored in the device or apparatus in a contactless manner and over a specified distance. A determination may also be made as to whether or not a particular object exists within a given reading range. Still other embodiments may provide information concerning physical parameters, such as a temperature or a pressure related to the monitored item in addition to providing a unique identification when direct access to the object is not possible.
According to other uses, a device or apparatus can, for example, be attached to an animal, which can then always be identified at an interrogation point, a threshold or feed location without direct contact. There is also a need for such a device, which when carried by a person, permits access checking, such that only persons whose transponder units return certain identification data to the interrogation unit are allowed access to a specified area. In this case, the safeguarding of the data transfer, such as by encryption, may be a very essential factor in the production of such devices. Still another example in which such a device is needed is the computer-controlled industrial production in which, without the intervention of operating personnel, components may be taken from a store, transported to a production location and there assembled to give a finished product. In this case, a device is required, which can be attached to the individual components so that the components can be specifically detected in the storage location and such that removal is monitored and recorded. Typically, most of the parameters of coded data transmitted to an interrogation unit have been previously determined such that the interrogation unit has been adjusted and the received signal is a sufficiently robust signal that it can be clearly identified from noise and spurious signals having a frequency within the frequency bandwidth of the transmitted data. However, as was mentioned above, if such RFID devices are to be used extensively, they are preferably inexpensive to manufacture and may be substantially disposable one-use devices. Therefore, to keep costs low many of these inexpensive transponder devices do not use a battery or other active power source, but instead may simply comprise a capacitor that collects and stores RF energy transmitted from the interrogation unit. This stored energy is then used for providing the identifying data transmission. Even if the device does include a small active power source, the power source may also be needed for data collection and data processing by the RFID device. Therefore, the power level of the transmitted data may be very low. In addition, if typical preliminary setup data bits or the “preamble” are eliminated, both the power usage and the time to complete an interrogation cycle are reduced. Unfortunately, simply eliminating the preamble renders the transmission susceptible to noise such that a transmitted message may be lost or received inaccurately.
Therefore, it would be advantageous to provide an effective technique for the interrogation unit to receive the transmitted data stream and accurately decode the data without the transmission of a preamble or preliminary setup and/or calibration data bits.